In situ monitoring of petroleum in soils is done using two commonly used techniques. One is direct sensing of petroleum thickness in wells with an oil-water interface probe (OWIP). In this method, a sensor attached to a tape measure is lowered into the petroleum-containing well. The sensor indicates (typically by emitting a beeping sound) when it reaches the air-petroleum and the petroleum-water interfaces.
Another in situ petroleum monitoring technique is a one-time survey using laser induced fluorescence (LIF). This technique uses a LIF sensor mounted to a drill probe. The LIF probe shines a UV laser and detects the fluorescence response of the petroleum hydrocarbon in the soil the probe penetrates. This technique was developed by the US Army Core of Engineers and was patented in the early-mid 1990s. The technique is currently commercialized by Dakota Technologies, Inc. (DTI) and the LIF one-time survey equipment are available therefrom. At least two other commercial enterprises hold licenses from DTI: Matrix and Columbia Technologies.
The disadvantage of the OWIP method is that the well itself acts as the largest pore in the ground, and tends to accumulate much more petroleum than the geologic formation, thus resulting in an inaccurate measurement. FIG. 1 shows the results of a sand tank experiment in which a simulated well (against the sand tank glass window) show both the petroleum thickness in the well and the petroleum in the formation. In this experiment, the petroleum is diesel with a small concentration of fluorescent dye tracer. There is a large difference between the petroleum well thickness (what the OWIP method would sense) and the actual distribution in the formation. The petroleum in the well does not reflect the immobile LNAPL below the water table (surrounded by water), nor the residual saturations in the vadose zone (the mostly air-filled part of the soil column above the water table). Additional effects such as heterogeneities in the soil can exacerbate these differences. For example, petroleum confined between fine lenses of fine pore materials, such as clay, can show up in wells even though there is none in the formation at the water table.
LIF is an widely used method tested by the EPA's Technology Verification Program. The limitation of LIF is that it requires drilling with every measuring event. The implications are that it is a destructive method (each bore hole can only be used once) and that it requires large equipment and personnel mobilization efforts associated with every event of drilling and sampling. It can be coupled with other high resolution measurements, with the associated devices attached to the drilling equipment point (such as, for example, a hydraulic profiling tool or membrane interface probe). FIG. 2 shows a schematic diagram of the equipment used (http://www.columbiatechnologies.com/services/#HRVP)
There is a need in the art for an improved method, system, and device for measuring in situ saturations of petroleum and non-aqueous phase liquid (NAPL) in soils.